This invention relates to the protection of swine, especially growing pigs, against swine dysentery infection by administration of killed cells of virulent isolates of Treponema hyodysenteriae (T. hyo.). In particular, this invention relates to a method of improving the effectiveness of oral administration of T. hyo. cells for this purpose.
Hudson et al found that oral dosing of an attenuated strain of T. hyodysenteriae provided no protection against subsequent challenge. Hudson, M. J., Alexander, T. J. L., Lysons, R. J., Wellstead, P. D., Brit. Vet. J. (1974) 130:37. Subsequently, Hudson et al attempted to immunize pigs with live attenuated T. hyodysenteriae using a combination of oral dosing and parenteral inoculation. Hudson, M. J., Alexander, T. J. L., Lysons, R. J., Prescott, J. F., Res. Vet. Science (1976) 21:366. Oral doses were administered on three consecutive days, and after an interval of several days, intraperitoneal vaccinations were administered, which were followed after several more days by intramuscular vaccinations. The overall results of these tests were summarized as follows: "Although vaccination appeared to enhance immunity to swine dysentery, half of the vaccinated pigs developed the disease. This level of protection would be unlikely to be practical value in the field."
Attempts to successfully utilize the oral administration of T. hyo. cells for increasing the resistance of swine to swine dysentery have continued. U.S. Pat. No. 4,152,413 described an oral preparation in which the T. hyo. cells are contained within enterically coated pellets. This reference teaches that the enteric coating should be selected so that it will be resistant to dissolving in the swine's stomach while dissolving in the swine intestines. U.S. Pat. No. 4,152,415 proposes that the enteric coated vaccine of U.S. Pat. No. 4,152,415 should be used in a combined parenteral and oral immunization regimen. One or more sensitizing doses of the T. hyo. cells are first administered parenterally, and after a delay of at least 5 days, the enteric-coated T. hyo. pellets are fed on a daily basis for period of from 5 to 8 days, or longer. The combined immunization is completed before the swine contract the infection.
Subsequent work leading to the present invention has shown that the use of enteric-coated pellets of T. hyo. does not always give reliable results, even if combined with a sensitizing parenteral injection as described in U.S. Pat. No. 4,152,415. While the enteric coating can be designed to effectively protect the T. hyo. cells in the stomach, there appears to be uncertainty as to when the cells are released from the coated pellets to achieve their immunizing action in the intestines. Although not known with certainty, it is now believed that the principal immunizing action is in the small intestines, and it seems likely that the enteric coatings are not dissolved rapidly enough for full immunizing action following tranfer of the pellets from the stomach to the small intestines. Further, the preparation of enteric-coated pellets is a relatively expensive procedure, which it would be desirable to avoid, if a method could be found which provided effective oral immunization without the use of enteric coatings.